Depending upon the type of drain in which a trap is to be installed, as well as upon its environment (e.g. whether it is located in the open or inside a dwelling), different requirements may have to be satisfied. A simple odor trap, for example, need only insure the continuous existence of a pool of water blocking the escape of sewer gases. In cases where backup of sewage is a problem, a manually operable shut-off valve may have to be provided for emergency use. Where the runoff into the trap is essentially from rainwater or melting snow, it may be advantageous to equip the trap with a catch basket for pebbles or the like swept along by the flow. Situations may also arise in which a device of this kind, originally designed just as a collection vessel, may have to be subsequently equipped with an odor-blocking insert or a shut-off valve.
In the case of an outside drain located close to a building wall, a trap needs to be disposed only slightly below the surface unless the precipitation to be carried off by its drain is collected from a larger area by the usual eave gutter and an associated leader terminating at or above that trap. If such a leader is installed after the trap has been emplaced, it may be necesssary to deepen the well in which the trap is located so as to reduce the risk of water accumulation on the surface which could result in the flooding of a nearby doorway. In that instance the pool formed by the trap may have to lie at a level between 1 and 11/2 meters below the surface; this makes it difficult to remove the usual lid of the trap in order to gain access to its interior for cleaning or replacing some of its parts.